events&delegates

8/3/2019 Events&Delegates

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What is a Delegate ?

Delegate is a type which holds the method(s) reference in an object. It is also referred to as a type safe

function pointer.

Advantages

Encapsulating the method's call from caller

Effective use of delegate improves the performance of application

Used to call a method asynchronously

Why Delegates?

In short the problem is that there is a tight coupling of function names with the UI client. So how can

we solve this problem?. Rather than referring to the actual methods in the UI / client if we can refer an

abstract pointer which in turn refers to the methods then we can decouple the functions from UI.

Later any change in the class ClsMath will not affect the UI as the changes will be decoupled by theAbstract pointer. This abstract pointer can be defined by using delegates. Delegates define a simple

abstract pointer to the function / method.


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Declaration

public delegate int mydelegate(int delvar1,int delvar2)

Note

You can use delegates without parameters or with parameter list

You should follow the same syntax as in the method

(If you are referring to the method with two int parameters and int return type, the delegate which you

are declaring should be in the same format. This is why it is referred to as type safe function pointer.)

Sample Program using Delegate

public delegate double Delegate_Prod(int a,int b);

class Class1

{

static double fn_Prodvalues(int val1,int val2)

{

return val1*val2;

}

static void Main(string[] args)


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{

//Creating the Delegate Instance

Delegate_Prod delObj = new Delegate_Prod(fn_Prodvalues);

Console.Write("Please Enter Values");

int v1 = Int32.Parse(Console.ReadLine());

int v2 = Int32.Parse(Console.ReadLine());

//use a delegate for processing

double res = delObj(v1,v2);

Console.WriteLine ("Result :"+res);

Console.ReadLine();

}

}

Explanation

Here I have used a small program which demonstrates the use of delegate.

The delegate "Delegate_Prod" is declared with double return type and accepts only two integer

parameters.

Inside the class, the method named fn_Prodvalues is defined with double return type and two integerparameters. (The delegate and method have the same signature and parameter type.)

Inside the Main method, the delegate instance is created and the function name is passed to the delegate

instance as follows:

Delegate_Prod delObj = new Delegate_Prod(fn_Prodvalues);

After this, we are accepting the two values from the user and passing those values to the delegate as wedo using method:

delObj(v1,v2);

Here delegate object encapsulates the method functionalities and returns the result as we specified in

the method.


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Multicast Delegate

What is Multicast Delegate?

It is a delegate which holds the reference of more than one method.

Multicast delegates must contain only methods that return void, else there is a run-time exception.

Simple Program using Multicast Delegate

delegate void Delegate_Multicast(int x, int y);

Class Class2

{

static void Method1(int x, int y)

{

Console.WriteLine("You r in Method 1");

}

static void Method2(int x, int y)

{

Console.WriteLine("You r in Method 2");

}

public static void Main()

{

Delegate_Multicast func = new Delegate_Multicast(Method1);

func += new Delegate_Multicast(Method2);

func(1,2); // Method1 and Method2 are called

func -= new Delegate_Multicast(Method1);

func(2,3); // Only Method2 is called

}

}

Explanation

In the above example, you can see that two methods are defined named method1 and method2 which


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take two integer parameters and return type as void.

In the main method, the Delegate object is created using the following statement:

Delegate_Multicast func = new Delegate_Multicast(Method1);Then the Delegate is added using the

+= operator and removed using the -= operator.

Events and Delegates

An event is a message sent by an object to signal the occurrence of an action. The action could be

caused by user interaction, such as a mouse click, or it could be triggered by some other program logic.

The object that raises the event is called the event sender. The object that captures the event andresponds to it is called the event receiver.

In event communication, the event sender class does not know which object or method will receive

(handle) the events it raises. What is needed is an intermediary (or pointer-like mechanism) betweenthe source and the receiver. The .NET Framework defines a special type (Delegate) that provides thefunctionality of a function pointer.

A delegate is a class that can hold a reference to a method. Unlike other classes, a delegate class has a

signature, and it can hold references only to methods that match its signature.While delegates have

other uses, the discussion here focuses on the event handling functionality of delegates. A delegatedeclaration is sufficient to define a delegate class. The declaration supplies the signature of the

delegate.

public delegate void AlarmEventHandler(object sender, AlarmEventArgs e);

The syntax is similar to that of a method declaration; however, the delegate keyword informs thecompiler that AlarmEventHandler is a delegate type. By convention, event delegates in the .NET

Framework have two parameters, the source that raised the event and the data for the event.

An instance of the AlarmEventHandler delegate can bind to any method that matches its signature, such

as the AlarmRang method of the WakeMeUp class shown in the following example.

public class WakeMeUp

{

// AlarmRang has the same signature as AlarmEventHandler.

public void AlarmRang(object sender, AlarmEventArgs e)

{

//...


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}

//...

}

References :

http://www.codeproject.com/KB/cs/Delegates_in_C_.aspx

http://www.codeproject.com/KB/cs/6ImportDelegatesAndEvents.aspx

http://msdn.microsoft.com/en-us/library/17sde2xt.aspx

http://msdn.microsoft.com/en-us/vjsharp/bb188664.aspx
http://www.codeproject.com/KB/cs/Delegates_in_C_.aspxhttp://msdn.microsoft.com/en-us/library/17sde2xt.aspxhttp://msdn.microsoft.com/en-us/library/17sde2xt.aspxhttp://www.codeproject.com/KB/cs/Delegates_in_C_.aspx

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